This invention relates to a process for the preparation of carbon fibers.
As carbon fibers are increasingly applied in many fields, an attempt is now being made to use then for incorporation into bodies of plastics, ceramics, concrete and metals. As precursor materials for carbon fibers, polyacrylonitrile fibers have been hitherto employed. Since the carbon fibers obtained from polyacrylonitrile fibers are expensive, however, a number of studies have been made in recent years to utilize relatively inexpensive carbonaceous pitch as raw materials for carbon fibers.
The general method for the production of carbon fibers from carbonaceous pitch includes melt spinning pitch into fibers, rendering the spun fibers infusible, and carbonizing the infusible fibers. To smoothly perform such a method, the properties of the raw material pitch are very important. The most important requirement is that the pitch must have a good spinnability. It is also important that the pitch must have properties so that the spun fibers obtained therefrom may be rendered infusible and carbonized without difficulty. Pitch which can meet with the above criteria has been hitherto considered to be of a type which is obtained by carefully thermally treating a raw material oil, such as a naphtha cracking residue, a recycle oil in fluidized bed catalytic cracking processes or a coal tar, which has a high content of aromatic components and a low content of impurities such as metals, inorganic matters and sulfur components. Thus, pitch derived from a low grade oil such as a vacuum residue or an atmospheric distillation residue has been considered to be unsuitable for use as a raw material pitch for the production of carbon fibers, since such a pitch has a poor spinnability. Since the high grade raw material oils described above are relatively expensive and fail to give pitch with a high yield, the carbon fibers obtained therefrom are also expensive. Therefore, there is a great demand for a process which can produce carbon fibers at a low cost.